A RAID technique has hitherto been used to improve the access performance and defect resistance of storage devices such as a hard disk. For example, with a RAID5 configuration, plural storing object blocks composed of plural data blocks and parity blocks are dispersedly stored in plural storage devices in blocks as a block group; the parity blocks are redundancy data used if any of the plural data blocks is defective, to rebuild the defective data block. For example, Patent Document 1 discloses a RAID5 configuration in which parity blocks are dispersedly stored in plural storage devices (Patent Document 1: Published Examined Patent Application No. 5-47857).
A technique has been disclosed which operates if one of plural storage devices is defective, to rebuild corresponding storing object blocks and store them in spare blocks (Patent Document 2: Published Examined Patent Application No. 7-24039). Furthermore, a technique has been disclosed which provides a first and second parity blocks and which updates either the first or second parity block upon a write so that the update of the parity blocks associated with a block write can be distributed to the first and second parity blocks (Patent Document 3: Published Unexamined Patent Application No. 9-34651).
Problems to be Solved by the Invention
If the technique indicated in Patent Document 2 or 3 is used, when one of the storage devices is defective, lost blocks must be rebuilt for all the block groups. The rebuilt blocks must then be written to spare blocks. In this case, a defect in one of the storage devices is critical because information stored in the storage system may be lost if further storage devices become defective. It is thus desirable to reduce overheads associated with the rebuilding of blocks.